Known glasses having optical characteristics such as a high refractive index (nd≧1.8) and good dispersion (vd≦30) include SiO2—Na2O—K2O—BaO—TiO2—Nb2O5 based glass (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-36103) and similar glass comprising B2O3 (U.S. Pat. No. 4,734,389).
One known method of manufacturing optical elements such as lenses is to manufacture an intermediate product called an “optical element blank” that is similar in form to the optical element and then grind and polish the intermediate product to obtain an optical element. Such methods of manufacturing intermediate products include the method of press molding a suitable quantity of glass melt to obtain the intermediate product (known as the “direct pressing method”); the method of casting glass melt into a glass plate in a casting mold, cutting this glass plate into multiple pieces of glass, reheating the glass pieces to soften them, and press molding the glass pieces into intermediate products; and the method of forming blocks of glass known as “glass gobs” from suitable quantities of glass melt, barrel grinding the glass gobs, reheating the glass gobs to soften them, and conducting press molding to obtain intermediate products. The method of reheating the glass to soften it and conducting press molding is referred to as the “reheat press method” to distinguish it from the direct press method.
All of these methods have in common the forming of glass while in a molten state followed by cooling, solidification to obtain a formed product, and machining of the formed product by processes such as cutting, grinding, and polishing. However, residual distortion resulting from the cooling process is present in the formed glass product obtained by forming while soft, and this product tends to be damaged during machining. Thus, it must be annealed prior to machining to reduce residual distortion.
However, when these methods are applied to the manufacturing of optical elements of glass having a high refractive index and good dispersion, there are unavoidable problems in that the glass devitrifies during the manufacturing process and the yield drops. In particular, when employing the reheat press method, devitrification of the glass is marked. This devitrified glass is unsuited to use in optical elements such as lenses.
That is, the press molding of those optical glasses with essential components in the form of SiO2, BaO, and TiO2 that have a refractive index (nd) of greater than or equal to 1.80 without devitrification is extremely difficult.
The first object of the present invention is to provide an optical glass having good resistance to devitrification, a high refractive index, and good dispersion; an optical element and a glass material for use in press molding comprised of this optical glass; and a highly efficient method of manufacturing optical elements employing this optical glass without causing the glass to devitrify.
There is also a method of pouring glass melt into a casting mold to form glass plates, annealing the glass plates, cutting the glass plates into cubes to obtain cut pieces, and grinding the glass pieces in a barrel grinder to obtain a material for press molding. This material is softened by heating and molded in what is known as “reheat pressing” to obtain molded products such as lens blanks. The lens blanks can then be ground and polished to manufacture lenses.
Almost all optical glasses serving as materials for lenses and the like yield optical element blanks of good quality by reheat press molding. However, depending on the type of glass, there are cases where, even when transparent material for press molding is employed, the molded glass devitrifies and cannot be employed in optical elements even with grinding and polishing.
It is not known why conventional, transparent materials for press molding devitrify when molded, and no countermeasures have been developed.
Accordingly, the second object of the present invention is to provide a method of manufacturing glass materials permitting the manufacturing of transparent press molded articles of high quality even from glass tending to devitrify during heat press molding, and a method of manufacturing press molded articles.
The third object of the present invention is to provide a method of manufacturing glass materials in which a determination is made as to whether the glass material is one tending to devitrify during reheat press molding, and when determined to be a glass material tending to devitrify, converted to a glass material tending not to devitrify; and a method of manufacturing press molded articles.